In radio-frequency applications, for example in mobile radio, low noise preamplifiers are normally used at the input of a receiver's signal processing chain. Low noise preamplifiers are also referred to as LNAs (Low Noise Amplifiers). Such low noise preamplifiers provide a sufficiently high signal-to-noise ratio (SNR) for downstream signal processing blocks while contributing little noise.
Radio-frequency receivers with an LNA of this type are normally developed by adding a transmission path to produce “transceivers”. Hence, bidirectional transmission of data, for example of voice data, is possible.
In line with the mobile radio standard GSM (Global System for Mobile communication), for example, sending and receiving take place alternately. Since this alternate sending and receiving takes place without being noticed by the user, however, GSM mobile radios are nevertheless referred to as full duplex systems.
When a transceiver of this type is being used for useful data transmission with an active transmission path, then it is necessary to ensure that the input power applied to the input of the reception path, which is turned off, is not too high. This can firstly result in interference signals in the transmission spectrum but could secondly also result in premature ageing through to destruction of the nonactive input transistors in the low noise preamplifier in the receiver. For this reason, the input transistors in the nonactive amplifiers are turned off when the receiver is inactive, since they are not needed and furthermore the current drawn by such transceivers is normally a significant factor, particularly for use in mobile radio.
One possible way of ensuring sufficient attenuation of the transmission spectrum for the inputs of the preamplifier in the reception path is to connect external surface acoustic wave filters upstream of the preamplifier. These surface acoustic wave filters are often present anyway in order to ensure adequate channel selection in the reception path.
However, external surface acoustic wave filters represent a relatively high level of complexity for transceiver units which are otherwise designed largely using integrated circuitry, which means that a sufficiently high level of insulation between the reception path and an active transmission path is wanted even when individual external SAW filters are dispensed with. The aim is to be able to use chip modules which have a plurality of SAW filters integrated in them.